1. Field of the Invention
The present invention relates to a composition having a corrosion-inhibiting action, to its use and to a process for reducing active corrosion of steel reinforcement in concrete. The invention further relates to a concrete obtainable using such a composition.
2. Description of the Background
Although steel-reinforced concrete is generally very durable, concrete constructions suffer from visible damage which is attributable to corrosion of the steel reinforcement and sometimes occurs only a few years after the construction has been built, although the life of a steel-reinforced concrete construction is usually planned as 100 years. Visible damage on the steel reinforcement often occurs after as little as 20 years. Expensive maintenance work is the consequence. There is therefore a very great need for effective corrosion inhibitors for steel-reinforced concrete.
In the past, various approaches have been tried for this purpose:
Use is nowadays made mainly of corrosion-inhibiting concrete additives such as alkali metal or alkaline earth metal nitrates (U.S. Pat. Nos. 3,210,207, 3,801,338, 3,976,494, 4,092,109 and 4,365,999). Apart from the ecological and toxicological concerns associated with the nitrates, they have the disadvantage that they are soluble in water and can easily be leached from building materials comprising a mineral binder based on cement.
Furthermore, alkanolamines in admixture with trihydric or tetrahydric alcohols have been described (U.S. Pat. No. 4,726,914) for the protection of steel surfaces. However, the use of such materials in mineral building materials comprising a cement-based binder is not described.
EP 0 209 978 A2 describes water-miscible corrosion inhibitors based on hydroxylamines for protecting the steel reinforcement in concrete.
CH 686 368 likewise relates to water-soluble corrosion inhibitors based on hydroxylamines, but, in contrast to EP 0 209 978 A2, these are reacted with carboxylic acids to reduce their volatility before use as a corrosion inhibitor for steel-reinforced concrete. However, considering the high pH of cement-bonded building materials such as concrete, this method appears rather doubtful since, according to the generally held view among chemists, the free amine is formed again at high pH.
One thing that is beyond doubt is that hydroxylamines can reduce corrosion on steel surfaces. This is evidenced, for example, by the corrosion current measurements reported in the abovementioned documents.
Reduction in corrosion is generally only conceivable when corrosion inhibitor is added to binder. Thus, the corrosion inhibitor has to be used during production of the building material. A disadvantage of this is that the introduction of the corrosion inhibitor changes the rheological properties of the cement-bonded building material, e.g. concrete, which increases the complexity of present-day mixtures and is therefore generally not desirable. In addition, the water solubility and the vapor pressure of the substances used as corrosion inhibitors lead to undesirable mobility in the cement matrix.
In addition, the active ingredient can be lost by leaching. In the publication entitled xe2x80x9cMigrating Corrosion Inhibitor Blend for Reinforced Concrete Part 2xe2x80x94Inhibitor as Repair Strategyxe2x80x9d (Corrosion, vol. 56, No. 7, page 727 ff.), it was found that the action of water-soluble corrosion inhibitors as described in CH 686 368 on the surface of steel present in a cement matrix does not necessarily have a positive effect on the measurable corrosion currents. The cause of this is very probably the fact that the corrosion inhibitor cannot diffuse sufficiently well through the cement matrix, so that a concentration of inhibitor sufficient for a corrosion-reducing effect cannot reach the steel surface, although the inhibitor could act in a high concentration (10%) over a number of weeks. Such long times for the action are also impractical. In the surface treatment of concrete, for example in the case of waterproofing measures for protecting constructions, the contact times range from only a few minutes (liquid impregnation compositions) to a few hours (paste-like impregnation compositions).
It may be said that there is at present no corrosion inhibitor for steel reinforcement in cement-bonded mineral building materials (concrete) which in the case of existing structures can be applied to the surface of the building material and prevents corrosion of the steel reinforcement in the presence of existing active corrosion.
It is therefore an object of the invention to provide a corrosion inhibitor for steel-reinforced concrete which can, if appropriate, be applied to existing concrete structures or can be added to the concrete mixture and effectively reduces corrosion current at the steel reinforcement, particularly when chlorides and other harmful materials present have already triggered corrosion.
It has surprisingly been found that application of a preparation, i.e. a composition, based on alkylalkoxysilanes or alkylalkoxysiloxanes effects a significant reduction in the corrosion currents measured at the steel reinforcement in concrete, even when the concrete has already been damaged by chlorides which have penetrated into it.